Note that as of this commit, there aren't any such throwers, and the
call site in Heap::allocate will drop exceptions on the floor. This
commit only serves to change the declaration of the overrides, make sure
they return an empty value, and to propagate OOM errors frm their base
initialize invocations.
Instead of creating a new global object and proxying everything through
it, we now evaluate console inputs inside a `with` environment.
This seems to match the behavior of WebKit and Gecko in my basic
testing, and removes the ConsoleGlobalObject which has been a source of
confusion and invalid downcasts.
The globals now live in a class called ConsoleGlobalObjectExtensions
(renamed from ConsoleGlobalObject since it's no longer a global object).
To make this possible, I had to add a way to override the initial
lexical environment when calling JS::Interpreter::run(). This is plumbed
via Web::HTML::ClassicScript::run().
